Level is classified into liquid level and material level. The liquid level may be measured with a liquid level annunciator or through a continuous liquid level measurement. The liquid level annunciator is for measuring liquid levels at several fixed positions, and is applied in, for example, warnings for upper and lower limits of the liquid level. The continuous liquid level measurement, i.e., continuously measuring the liquid level is widely used in fields of, for example, oil, chemical industry and food processing.
At present, methods for continuous liquid level measurement include a hand-dipping method, a float ball method, a servo method, a static pressure method, a magnetostriction method, an ultrasonic method, an optical fiber sensor method, a capacitance method and so on. In practical applications, proper kinds of liquid level meters may be selected based on factors such as price, measurement accuracy and characteristics of the measured medium. Although with so many optional measurement methods, it is still difficult to satisfy requirements under some conditions.
For example, when applying the above methods in measuring the liquid level in a fuel tank of a diesel generator acting as a back-up source for telecommunication, the following problems may be caused.
1. Non-electricity measurement methods such as a glass tube method and the hand-dipping method require participation of humans and are not applicable for working conditions without the presence of humans.
2. The diesel fuel may generate cotton-shaped solid precipitates during storage, and the precipitates may accumulate to a thickness from 2 cm to 5 cm. In usage of a meter such as an optical fiber sensor or an immersion liquid level sensor, the sensor should be placed at the bottom of a container, and an elastic diaphragm for pressure transmission is liable to be blocked or damaged because of the solid precipitates, thereby resulting in, for example, poor pressure transmission. Therefore, problems such as liquid level fluctuation and large measurement bias may be caused in practical applications.
3. Fuel tanks of different volumes and shapes may have measurement ranges from 30 cm to 200 cm. The float ball method, dedicated for small measurement ranges, is not applicable for measuring the liquid level of the fuel tank. Meters having fixed measurement ranges, utilized in methods such as the magnetostriction method and the capacitance method, are inconvenient in transportation, installation and operation.
4. The diesel generator may operate with vibration. Methods such as a float bowl method or the servo method, in which sensors including moving mechanical parts are utilized, are not applicable under conditions with vibration.
As another application example, a floater method is widely used in measuring the liquid level of automobile fuel tank. A floater is sleeved on a straight tube, a magnetic ring is arranged in the floater, and multiple reed switches are arranged in the straight tube. The floater slides up and down along the straight tube based on the liquid level. The reed switch close to the floater is turned on in response to the influence of the magnetic ring in the floater, and accordingly, a liquid level signal is output. The floater method has a simple principle and a low cost, however, the liquid level measurement with this method is discontinuous and the moving mechanical part is of poor reliability.
An optical fiber pressure sensor may also be used for measuring the liquid level. The optical fiber pressure sensor has a structure as follows. In a housing, an end of one optical fiber is placed at a front end of an elastic diaphragm, and a reflection film is provided on the elastic diaphragm. When the liquid level changes, the elastic diaphragm moves, and accordingly, the reflection film moves. Change of the pressure (the liquid level) may be sensed based on change of the light intensity sensed by a probe. This kind of sensor is slender and small and may be made into a specific shape to match a measured component. However, the sensor also has disadvantages such as difficult manufacture and high cost. Furthermore, the sensor should be provided with the elastic membrane. Same as a diffusion silicon diaphragm capsule immersion liquid level sensor, the optical fiber pressure sensor is not applicable for performing measurement with the existence of solid precipitates in the fuel tank of the diesel generator.
With the rapid development of science and technology nowadays, a method for accurately measuring the liquid level and an apparatus thereof, which may be used under bad operating conditions and may not be influenced by specific factors, are required to measure the levels of liquids in various devices and apparatuses.